1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
A wireless communications system typically employs a plurality of base stations distributed over a geographic region and a plurality of mobile devices. The base stations are typically fixed, and the mobile devices are generally free to move about the geographic region, alternately communicating with one or more of the fixed base stations, depending on a number of factors, such as the quality of communications available with a particular base station.
In general, the overall strength of the total received signals measured in the base station is commonly represented as RSSI (Received Signal Strength Indicator). RSSI is generally comprised of at least two components, a noise floor component and a variable component (also known as noise rise or RSSI rise). The noise floor component of RSSI is the signal level that exists independent of any signals being exchanged between the mobile devices and the base stations. That is, the noise floor is the signal that exists even when the mobile devices and base stations are not communicating. The variable component, on the other hand, is the portion of the RSSI that is caused by the mobile devices communicating with the base station, and, significantly is an indication of reverse interference and is a function of reverse link loading caused by more than one mobile device transmitting signals at substantially the same time. For example, if mobile devices A and B transmit signals at the same time, the signal from mobile device A interferes with the signal from mobile device B, and vice verse. One factor that affects the amount of the variable component of RSSI within a base station is the number of mobile devices and data rates they are actively using to communicate with the base station. Thus, the variable component of RSSI is a useful indicator of reverse link loading, which directly affects capacity and performance of a wireless telephony system.
In a wireless system, a base station normally has multiple paths over which a particular signal may be received. For example, a particular signal may be reflected off of a number of landmarks (e.g., buildings, trees, vehicles, etc.) before reaching the base station. Thus, multiple reflections of the same signal may reach the base station. These reflected signals may be substantially identical, but varied in time and could experience different RF fading, owing to the different paths that they traversed. The multiple signals are useful to create diversity gain, and thus are commonly referred to as diversity signals. Each receive path may have its own RSSI measurement circuit. In the ideal case, the RSSI measurement from the various diversity signals may be the same, as all of the signals are sent with the same energy and with the same path loss.
However, each of the receive paths may have gain variations in base station receiver hardware arising from different gains/losses associated with different components in the receive chain. RSSI is normally measured after passing through many Radio Frequency (RF) components in the receive chain. Thus, any of these components could cause RSSI measurement variations. For example, an RSSI sample at diversity signal A could be −86dBm while the RSSI measurement at diversity signal B is −83dBm. In some extreme cases (e.g., one diversity signal is dysfunctional), the signal difference between two diversity signals could be as high as 20dB. This large differential presents a challenge to the RSSI processing. That is, how to deal with the absolute RSSI measurements from different diversity signals without compromising the RSSI measurement integrity while taking advantage of the multi-path gain.
In prior systems, the variable component of RSSI is calculated from the RSSI measurement and an estimated noise floor. Both are obtained from the RSSI measurements; however, the RSSI measurement can be taken from one diversity signal while the noise floor estimation can be based on another diversity signal. This presents an inconsistent view on the variable component of RSSI and impairs the accuracy of reverse link traffic loading control, leading to degraded reverse link performance on the air interface for high speed data services.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.